Heat Recovery Unit for shower

I love showers. I love long showers. Let's say I take an "hour power shower".

With that in mind I've been thinking of installing a heat recovery drain system for an upstairs shower. I'm currently thinking of "Power Pipe" but I'm sure there are others.

First, does anyone here have any experience with these good or bad. And second: Typically one would install this in line with the cold supply for the hot water tank but since my tank is a good distance from where the heat recovery unit would be, it would be simpler to install this in line with the showers cold supply. I'm not sure if I would need a fancy mixing valve to make this feasible. Any thoughts?

I am going to install a second hotwater tank in my mechanical room and strip it of any insulation. My plan is to use the warmth from the room to pre-heat the water overnight to about 24 degrees C.

When we have demand for hot water the water will first enter the secondary hot water (holding tank) and then feed the proper hot water tank. I think this will save me hundreds of dollars if not thousands over the next ten years.

An investment like this will take years to break even. Don't expect to see a return on your investment any time soon.

You need to rely on recommendations from the manufacturer about install options. Their engineers should have the answers to those questions.

What are you wanting to accomplish with this? Certainly some energy will be recovered. How much...check their research. Figure that the water cools considerably as soon as it starts running off your back. It gathers on the tile floor, and eventually into cold uninsulated drain pipes. So if you expect this system to give you longer showers, I don't think that is going to happen.

I assume you have a tankless water heater?, since if would take about a 200 gallon tank-type to provide an hour shower@!!

The recovery amount will be so miniscule that it is doubtful you would ever notice its effect. To get heat transfer heat source and the water have to be in contact long enough for the transfer to take place. Usually this is done with a twisted "labyrinth", a long transfer pipe, such as a flue, or multiple small tubes with a lot of surface area. Your shower unit will have none of these.

The in situ effectiveness testing looked at how long the shower could be run before the water temperature dropped below 37Â°C (98.6Â°F). All DWHR devices resulted in significantly longer hot water availability times than the benchmark time of 28 minutes. Configuration A results ranged from 39 minutes to 62 minutes. Configuration B results ranged from 53 to over 75 minutes.

On that link, the $$ figures are fairly impressive. I don't know what the units cost, but with installation, I see a 10 year payback.
Apparently the "technology" is that the drain water tends to flow in a thin film on the inner surface walls of the drain pipe, providing a lot of surface area for thermal transfer.

As with all research, the data raises as many questions as they answer. It seems they are using a starting shower temp of about 105Âº, but run until the shower temp drops to 98Âº?? That is not acceptable to most people. If anything, I personally tend to dial UP the temp after a bit of time. I normally take a pretty short shower, but if I feel like a "steamer" then 98 doesn't cut it!

All in all, it does look like there is some savings worth pursuing. The websites do seem to show that the system must be vertical, so retrofit may be difficult.

Heat transfer depends on numerous factors, but the delta T, or difference in temperature is a big factor. Water does tend to run down the sides of the pipe, even when vertical, rather than falling down the middle of it, so you end up with a thin film on the maximum surface area. Then, by splitting the supply water into multiple square channels like one of the recovery systems does, you tend to maximize the heat transfer. By warming the inlet water to the WH, you're decreasing the load and making it look like a larger WH. If you warm the cold into the shower, you could use slightly less hot and achieve the same temp, but you would have a much bigger variation and still likely have to start with a hotter input. This is a situation where a thermostatically controlled valve would have advantages, even as the hot cooled off, the valve would be adjusting the hot/cold mix to try to maintain the set temperature. I think I'd put it in the cold supply to the WH and then make sure I'd insulated all of the piping to retain the heat gained.

The recovery amount will be so miniscule that it is doubtful you would ever notice its effect. To get heat transfer heat source and the water have to be in contact long enough for the transfer to take place. Usually this is done with a twisted "labyrinth", a long transfer pipe, such as a flue, or multiple small tubes with a lot of surface area. Your shower unit will have none of these.

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Incorrect. This is the most common layman's misunderstanding of how heat transfer works: "in contact long enough." It's not the residence time but the heat transfer coefficient and area, along with the driving force (delta T). Liquid film coefficients are quite high.

If you look at the designs of the drainwater heat recovery units they can achieve reasonably good recovery. The layout is typically one of countercurrent, falling thin film exchanger, often with a multi-parallel path coil in close contact on the other side. Combine these with the excellent heat transfer properties of water and you have a functional design.

I evaluated a few designs a year or so ago the same way as I did when doing heat exchanger design including--thin film types--for a living (and I've not yet had any exchanger I designed/specified underperform.) I started out skeptical of some of the claims, but when I began reviewing the drawings I achieved similar results with my own calculations of heat transfer coefficients, pressure drop, effective area and such.

Higher volume/long duration shower users will get more benefit than low volume users. For high volume, minimizing pressure drop requires more parallel coils and heat transfer efficiency declines somewhat, but that is more than made up for by the greater total Btu's recovered.

I have three little girls and they all have bath and shower time after dinner. This is my biggest demand time for hot water.

What do you think of a second tank as a holding tank? I would think in a 24 hour period the holding tank would have enough hot water or at least warm water that the recovery of the main hot water tank would be quicker and shorter. And in doing so cheaper.

I have even thought of wrapping the spare tank with my hot water heat return legs. Tapping any heat out of them and making the temperature of the holding tank even warmer???

quote; Then, by splitting the supply water into multiple square channels like one of the recovery systems does, you tend to maximize the heat transfer.

Splitting it into "multiple square channels" may make it transfer heat better, but would be disastrous for a shower drain, especially when it comes time to clear the debris out of those channels. Did any of the evaluations consider the degradation caused by soap and hair buildup on the walls, which would be exacerbated by having small channels? Also, what were the respective flow rates? High flows would not allow the "hot" and "cold" waters to stay in contact long enough to get the temperature differentials the diagram shows. The discussion might be academic, because a search for "Watercycle" does not produce any results so they may not be a factor any longer.

Wow. I had no idea this forum was so widely read. Thank you to all who read and replied.

Admittedly I don't take "hour power showers" but I liked the rhyme.

More accurately, I am planning a new home in a rural location on Vancouver Island so my water will be drawn from a chilly well. Without natural gas, BTUs for water heating are relatively expensive so my payback on a DWHR unit will likely be shorter than that for the majority of North Americans.

The main shower in the house presents and ideal installation as its drain immediately drops one story to the main drain below grade. Because the concept works best when drain flow matches inlet flow I wouldn't bother re-designing the main stack to accommodate one of these as the shower is the only load in my house that matches the basic criteria.

The only question in my mind is can I use the drain waste heat recovery unit to warm the shower's cold water supply or with this present a problem for the mixing valve?

Jim, You seem to suggest that I could use a thermostatic mixing valve to do this but your preference would be to warm the water heaters supply. Having thought about it more I tend to agree based on the my belief that, after the shower is over, the DWHR unit will have some latent BTUs that might get used the next time there is a call for hot water. Perhaps minuscule but the additional plumbing is not worth mentioning.

Thanks again all!

The following are some links for others considering drain waste heat recovery.

Splitting it into "multiple square channels" may make it transfer heat better, but would be disastrous for a shower drain, especially when it comes time to clear the debris out of those channels. Did any of the evaluations consider the degradation caused by soap and hair buildup on the walls, which would be exacerbated by having small channels? Also, what were the respective flow rates? High flows would not allow the "hot" and "cold" waters to stay in contact long enough to get the temperature differentials the diagram shows. The discussion might be academic, because a search for "Watercycle" does not produce any results so they may not be a factor any longer.

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For clarification, it is the water supply line that is split into multiple channels not the drain. The drain portion is simply a straight cooper pipe with an i.d. of 2", 3", 4", or 6" (Available PowerPipe sizes)

It seems that most of the references I have found are Canadian so payback may be climate dependent and not applicable to your part of the world. I suspect that your cost per BTU is somewhat cheaper as well.

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What do you think of a second tank as a holding tank? I would think in a 24 hour period the holding tank would have enough hot water or at least warm water that the recovery of the main hot water tank would be quicker and shorter. And in doing so cheaper.

I have even thought of wrapping the spare tank with my hot water heat return legs. Tapping any heat out of them and making the temperature of the holding tank even warmer???

JW

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Your recovery time may be shorter but I can see no cost savings here. It strikes me as an inefficent use of space and money. Given that your "holding tank" will be absorbing BTUs from a conditioned space, a space that you paid to condition, there is no net gain in BTUs. You're not capturing lost energy, you're mearly transfering energy.

Your money might be better spent investigating solar domestic hot water. The District of North Vancouver has already done you the favour of estimating your savings based on your location's solar exposure. See their mapping applications at http://geoweb.dnv.org/

Here are the stats for your house.

Solar BC has grants available and I expect you have the expertise to install it.

Splitting it into "multiple square channels" may make it transfer heat better, but would be disastrous for a shower drain, especially when it comes time to clear the debris out of those channels. Did any of the evaluations consider the degradation caused by soap and hair buildup on the walls, which would be exacerbated by having small channels?

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Please look at the actual design. The drain water is flowing down the walls of the drain pipe not in the quad coil, single coil or dual coil. This produces a relatively thin film. Being vertical and open it is somewhat self cleaning. Soap scum and the like will likely reduce heat transfer only slightly over time unless the build up is substantial. However, it is a valid concern and one reason that I would want the ability to remove the assembly for cleaning.

An area of more concern to me, but related to any build up is maldistribution of liquid in the falling film. Maldistribution is a factor that many old school engineers never adequately considered, so it is one of the first things I examine for problems in designs. However a mitigating factor in this design is the nature of the flow and the relatively high conduction rate of the copper should compensate for a fair amount of maldistribution. If some manufacturer wants to test this I've got a good location in mind as long as I get to keep the unit when I'm done. Wouldn't be hard to test off-level installation, some simulated hanging hair balls, or soap build up that create maldistribution.

High flows would not allow the "hot" and "cold" waters to stay in contact long enough to get the temperature differentials the diagram shows.

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Contact time, residence time or whatever one calls it is not a relevant criteria for heat transfer. By itself it is meaningless. In fact, this greater contact time the average person think improves heat transfer in fact IMPEDES it. Film heat transfer coefficients increase as velocity increases. So for a given surface area higher velocity = higher heat transfer coefficient = lower contact time/residence time.

One uses more surface area/smaller flow channels where possible to increase velocity. Surface area is typically limited by material/fabrication costs or geometry as well as pressure drop. Decreasing flow channel size or increasing length is limited by pressure drop if not by geometry or plugging concerns.

The square quad coil design is attractive because it keeps pressure drop in control in greater overall drain pipe lengths and with higher flowrates (as in several showers or other users drawing water at the same time.) The squarish coils are needed to provide good contact with the pipe, something round coils don't achieve--this I can attest to after seeing too many jerry-rigged tubing coils on pipes in plant settings.

More accurately, I am planning a new home in a rural location on Vancouver Island so my water will be drawn from a chilly well.

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The water from the hole itself should be relatively unchanged throughout the year, but by the time it runs through the well tank and piping in the ground it, of course, is much cooler in winter than summer. But the longer the winter and shorter the summer, the greater the impact.

Being on a well (or any system where supply press. to the home might not be all that great), pay careful attention to pressure drop in the device at anticipated maximum numbers of hot water consuming fixtures running simultaneously. No, not everything turned on at once, but say two showers running at the same time and possibly a sink faucet, maybe the clothes or dishwasher as well. In that case the well supply pressure might be a bit low too and then there is the pressure drop to the house. I'm on city water and have to use a PRV to keep the pressure down, but I would still size the system to not drop the hot water tap pressure too much with all three of our showers running simultaneously (which we do once in awhile.)

The main shower in the house presents and ideal installation as its drain immediately drops one story to the main drain below grade. Because the concept works best when drain flow matches inlet flow I wouldn't bother re-designing the main stack to accommodate one of these as the shower is the only load in my house that matches the basic criteria.

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Same basic issue for me. 2 of 3 showers in our home would be served by this, serving 3 of the 4 occupants and over 75% of the showering.

The only question in my mind is can I use the drain waste heat recovery unit to warm the shower's cold water supply or with this present a problem for the mixing valve?

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Yes, you can. That is the preferred way efficiency wise because it will increase the overall heat extraction because the delta T at the outlet will be greater (even with greater heat recovery) than it would be at a lower cold water flow rate. Q = U*A*LMTD Where Q = dury, U = overall heat transfer coefficient, A = area, LMTD = log mean temperature difference.

For simplicity sake just think of LMTD as the average delta T at both ends of the heat exchanger. (The formula is slightly more complex but this is close enough for understanding.) When you increase the cold water flow at a given duty the hot water outlet temperature wouldn't change, but the cold water temperature rise would be less than before. In reality the duty is not fixed, but area is, as is the drainwater supply temp. (U will increase slightly because of the flow increase, but since the tube side coefficient is probably not the limiting side, it won't have as much impact.) The LMTD increases, so the duty also increases.

The one negative of having the shower cold water leg go through the heat recovery system is that as the system warms up, your "cold" water supply to the shower will be warming as well, so you will likely want to adjust the mix of hot/cold manually during the shower. Other hot water users cycling on/off during the shower will impact this some too. So some sort of thermostatic control would be desirable for this (particularly for children...or a cranky spouse.)

A thermostatic shower valve adjusts how much cold is used to dilute the hot. It tries to keep the preset temperature constant. If either the cold temperature or the hot temperature changes, your outlet temp on a conventional valve will change. If you take a long shower, the cold will become colder and the hot will cool off. The only valve that will keep the temperature relatively constant for you is a themostatically controlled one. To me, worth the increased cost. It can never get hotter than the hotter input, and if the cold side got hot, it would rise to the cold temp as it turned off all of the hot.

I have three little girls and they all have bath and shower time after dinner. This is my biggest demand time for hot water.

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I have two suggestions for you that I used with my children. The first is to get one of those stick on 5 min hourglass "shower coach" thingies from the orange box store or another place. This works even if you give them 10 mins/each. The kids like playing with them anyway...so it wasn't hard to get mine to use them.

The second is to consider a lower flow showerhead for their shower. The two I like are in the 1.5-1.6 gpm range. I use the Evolve Roadrunner in my shower and one of the kids' showers and we have the High Sierra in another. I researched this for awhile before selecting these to try. They are both non-aerated designs that gave good full spray patterns (rather than hollow cone patterns.) An aerated design can give the impression of fuller flow, but also results in more mist generation requiring somewhat greater hot water mix for the same temp. Some of the ultra low flow aerated designs also have more of a needle like impingement sensation to them from what I've read.

Your recovery time may be shorter but I can see no cost savings here. It strikes me as an inefficent use of space and money. Given that your "holding tank" will be absorbing BTUs from a conditioned space, a space that you paid to condition, there is no net gain in BTUs. You're not capturing lost energy, you're mearly transfering energy....

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I do not pay to heat the space it is my mechanical room.

If I use the return loops to further heat the secondary tank I'm actually get more for my money anyway since I'm told my Loch N Var boiler will fire at the same rate to heat water at 10 degrees as it does 18 degrees.

If my kids take two baths and one shower a day this can easily be 40-60 gallons of hot water. If while they are filling the tub or showering and the hot water tank is getting feed by warm water I would think the recovery time and the amount of heating energy used would be far less.

I could pick up a spare tank for under $400.00. Maybe $40.00 in fittings. 2 beer on the weekend. Done.

I would see a return on this investment in less than two years I would think. Maybe sooner.